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AD9020PCB Folha de dados(PDF) 8 Page - Analog Devices |
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AD9020PCB Folha de dados(HTML) 8 Page - Analog Devices |
8 / 12 page AD9020 –8– REV. A USING THE AD9020 Voltage References The AD9020 requires that the user provide two voltage refer- ences: +VREF and –VREF. These two voltages are applied across an internal resistor ladder (nominally 37 Ω) and set the analog input voltage range of the converter. The voltage references should be driven from a stable, low impedance source. In addi- tion to these two references, three evenly spaced taps on the re- sistor ladder (1/4REF, 1/2REF, 3/4REF) are available. Providing a reference to these quarter points on the resistor ladder will im- prove the integral linearity of the converter and improve ac per- formance. (AC and dc specifications are tested while driving the quarter points at the indicated levels.) The figure below is not intended to show the transfer function of the ADC, but illus- trates how the linearity of the device is affected by reference voltages applied to the ladder. Effect of Reference Taps on Linearity Resistance between the reference connections and the taps of the first and last comparators causes offset errors. These errors, called “top and bottom of the ladder offsets,” can be nulled by using the voltage sense lines, +VSENSE and –VSENSE, to adjust the reference voltages. Current through the sense lines should be limited to less than 100 µA. Excessive current drawn through the voltage sense lines will affect the accuracy of the sense line voltage. The next page shows a reference circuit which nulls out the off- set errors using two op amps and provides appropriate voltage references to the quarter-point taps. Feedback from the sense lines causes the op amps to compensate for the offset errors. The two transistors limit the amount of current drawn directly from the op amps; resistors at the base connections stabilize their operation. The 10 k Ω resistors (R1–R4) between the volt- age sense lines form an external resistor ladder; the quarter point voltages are taken off this external ladder and buffered by an op amp. The actual values of resistors R1–R4 are not critical, but they should match well and be large enough ( ≥10 kΩ) to limit the amount of current drawn from the voltage sense lines. The select resistors (RS) shown in the schematic (each pair can be a potentiometer) are chosen to adjust the quarter-point volt- age references, but are not necessary if R1–R4 match within 0.05%. An alternative approach for defining the quarter-point refer- ences of the resistor ladder is to evaluate the integral linearity error of an individual device, and adjust the voltage at the quarter-points to minimize this error. This may improve the low frequency ac performance of the converter. Performance of the AD9020 has been optimized with an analog input voltage of ±1.75 V (as measured at ±V SENSE). If the ana- log input range is reduced below these values, relatively larger differential nonlinearity errors may result because of comparator mismatches. As shown in the figure below, performance of the converter is a function of ±V SENSE. 32 38 44 50 56 62 10.0 9.0 8.0 7.0 6.0 5.0 0.4 0.8 1.0 1.4 1.8 2.0 ±VSENSE – Volts 0.6 1.2 1.6 AD9020 SNR and ENOB vs. Reference Voltage Applying a voltage greater than 4 V across the internal resistor ladder will cause current densities to exceed rated values, and may cause permanent damage to the AD9020. The design of the reference circuit should limit the voltage available to the references. Analog Input Signal The signal applied to ANALOG IN drives the inputs of 512 parallel comparator cells (see Equivalent Analog Input figure). This connection typically has an input resistance of 7 k Ω, and input capacitance of 45 pF. The input capacitance is nearly constant over the analog input voltage range, as shown in the graph which illustrates that characteristic. The analog input signal should be driven from a low distortion, low noise amplifier. A good choice is the AD9617, a wide band- width, monolithic operational amplifier with excellent ac and dc performance. The input capacitance should be isolated by a small series resistor (24 Ω for the AD9617) to improve the ac performance of the amplifier (see AD9020/PCB Evaluation Board Block Diagram). |
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